Subject to Revision. 


[TRANSACTIONS OF THE AMERICAN INSTITUTE OF MINING ENGINEERS.] 


THE HYDROMETALLURGY OF COPPER , AND ITS SEPARA¬ 
TION FROM THE PR EC 10 VS METALS. 


BY T. STERRY HUNT, LL.D., F.R.S., MONTREAL, CANADA. 


(Read at the Virginia Meeting, June, 1881.) 


Wet processes for the extraction of copper from its ores have of 
late attracted much attention, especially in Europe, where the use 
of cupriferous iron-pyrites as a source of sulphur prevails. These 
processes present an additional interest, when, as is often the case, 
the copper-ores to be treated contain one or both of the precious 
metals, since the complete separation of these from copper, either in 
the dry or the moist way, is, as is well known, one of the difficult 
problems of metallurgy. The principal wet processes hitherto in 
use for the extraction of copper from its ores may be included under 
three classes: 

I. Those in which the copper in sulphuretted ores is rendered 
soluble in water by calcining them, after a preliminary roasting 
which removes the greater part of their sulphur, with a portion of 
common salt, by which the copper is converted into a chloride (Long- 
maid and Henderson), or with sulphate of soda, by which sulphate 
of copper is formed (Monnier). Allied to these is the method by 
which a portion of the copper is got as soluble sulphate by roasting 
without addition, a process which is sometimes modified and rendered 
more complete by re-roasting the lixiviated residues with the addition 
of a portion of raw sulphuretted ore (Bankart). 

II. Those methods in which free chlorhydric or sulphuric acid is 
used to dissolve the copper from oxydized or roasted ores. These, 
while simple and efficient, are too costly, except in certain localities 
where chlorhydric acid is a waste product. Related to them is the 
plan which consists in exposing the oxydized and moistened ores to 
the slow action of sulphurous acid mixed with air, by which a 
soluble sulphate of copper is formed. It has been proposed to de¬ 
compose the solutions of sulphate or chloride of copper by sulphur- 
retted hydrogen, thus separating the copper as sulphide, and using 
the liberated acid to dissolve fresh portions of oxydized copper. It 


1 





2 


HYDROMETALLURGY OF COPPER. 



is, however, the almost universal practice to throw down the copper 
from its solutions, however obtained, in the metallic state by the use 
of iron, either scrap-iron or iron-sponge, although in some instances 
milk of lime has been used as a precipitant, by which oxide of copper 
is produced. 

III. The method in which a hot solution of ferrous chloride, with 
common salt, is used to chloridize the oxydized copper and convert 
it into a mixture of cupric and cuprous chlorids, which latter, though 
nearly insoluble in water, is dissolved by help of the chloride of 
sodium. From this solution the copper is precipitated by metallic 
iron, thereby reproducing the ferrous chloride, and regenerating the 
solvent, an advantage which this process possesses over any other 
which gives the copper in the metallic form. This, which is known 
as the Hunt and Douglas method, may, in many cases, be used with 
advantage for the treatment of low-grade copper ores, and, as the 
neutral solvent does not dissolve either arsenic or antimony, enables 
fine copper to be got directly from ores holding these impurities.* 

When, however, it is applied to the treatment of copper-ores con¬ 
taining silver, the use of the bath of ferrous chloride and chloride of 
sodium has, in common with the method of roasting with salt, the 
disadvantage that it converts the silver into a chloride which is 
soluble in a strong solution of chloride of sodium and is then with 
difficulty separated from the chlorides of copper. The separation of 
silver and copper when dissolved together in the state of chlorides 
has greatly exercised the ingenuity of metallurgical chemists from 
its importance in connection with the treatment of Spanish and 
Portuguese pyrites, now so extensively used in Great Britain as a 
source of sulphur, where their consumption in 1877 amounted to 
600,000 tons. These ores contain on an average about forty-nine 
per cent, of sulphur and two or three per cent, of copper, with a 
little silver, equal, according to Claudet, to from twenty to twenty- 
eight grams to the ton of ore, and traces of gold. The residues 
after calcination still retain three or four per cent, of sulphur and 
about four per cent, of copper, with sixty per cent, or more of iron, 


* The hydrous silicate of copper (chrysocolla) is, like the carbonates of copper, 
completely decomposed by a hot solution of ferrous chloride with common salt. 
An account of the application of this method to the treatment of a crystalline 
hydrous silicate of alumina, magnesia, and copper (which is essentially a copper- 
chlorite, and has been described by the writer as a new species under the name 
of venerite), will be found in the Transactions of the American Institute of 
Mining Engineers, vol. iv, p. 328. 



THE HYDROMETALLURGY OF COPPER. 


3 


chiefly as peroxide. This material, after having been again calcined 
for some hours at a low heat in a reverberatory with about fifteen per 
cent, ot common salt (or, as at Oker in Germany, with crude chloride 
of potassium), gives up to water acidulated with a little chlorhydric 
acid its sulphur as sulphate of soda, and its copper and silver as 
chlorides, together with a considerable excess of common salt, leaving 
behind a nearly pure peroxide of iron with not over two thousandths 
ot copper. A minute portion of gold, which was converted into 
aurous chloride during the roasting, is also present in the solution.* 
The lixivium, in an example given by Claudet, had a specific gravity 
of 1.24, and held in a metre-cube, besides 144.0 kilograms of sul¬ 
phate of soda, 64.0 of chloride of sodium, and 52.8 of copper as pro¬ 
tochloride, besides small portions of zinc and iron, with a little lead, 
and 44 grams of silver. The above details of the salt-roasting pro¬ 
cess, which have often been published, are here repeated in order to 
bring more clearly before us the problem of separating the silver 
from the copper. 

Various plans have been suggested for extracting from such solu¬ 
tions as the above the dissolved silver before throwing; down the 
copper by metallic iron. It has been proposed to precipitate the 
silver by finely-divided metallic copper, as is done in the Augustin 
process from solutions of chloride of sodium holding only dissolved 
silver-chloride; but, since metallic copper at once converts proto¬ 
chloride into dichloride of copper, it becomes necessary, as a prelimi¬ 
nary to the precipitation of the silver by this means, that the whole 
of the copper in the solution should first be brought into the latter 
condition. This may be effected by treating the hot solution with 
sulphurous acid, or by filtering it at a temperature near the boiling 
point through a layer of coarsely-ground copper matte, or of vitreous 
or purple copper ore, from which, as I have found, a second equiva¬ 
lent of copper is taken up. From solutions holding the whole of 
the copper as dichloride, the silver may be readily thrown down in 
the metallic state by filtering them through a layer of finely-divided 

* Plattner, as is well known, showed that gold, like silver, is chloridized when 
pyritous ores containing it are roasted at a low red heat with common salt; 
an aurous chloride being apparently found, which, in the presence of chloride of 
sodium has a considerable degree of stability, though it is decomposed at higher 
temperatures. This chloride of gold is insoluble in water, and, unlike chloride of 
silver, is not decomposed by mercury. It is but slightly soluble in brine, though 
readily dissolved by a solution of hyposulphite of soda or lime. A process pro¬ 
posed b} T Kiss for the simultaneous extraction of silver and gold from pyritous 
ores is based on these reactions 





4 


THE HYDROMETAI/LURGY OF COPPER. 


metallic copper; but, inasmuch as the dichloride requires to hold it 
in solution, a great volume of hot concentrated brine,* this method 
of separating silver from solutions containing a considerable amount 
of copper is not in all cases practicable. 

Among the plans which have been proposed for the separation of 
the silver from these saline solutions are those based on fractional 
precipitation. This is effected, as at Oker, by the use of sulphide 
of sodium; or better, by diluted sulphuretted hydrogen gas, as 
got by the action of air mixed with carbonic acid on the waste cal¬ 
cium-sulphide from Leblanc’s soda-process. In the latter case, the 
whole of the silver is, according to Gibbs, carried down with 
the first six per cent, of the copper. Snelus blows finely-divided 
metallic iron into the solution, and finds the first twenty per cent, 
of copper thus precipitated holds four-fifths of the silver. For the 
extraction of the silver from the mixed sulphides it suffices to con¬ 
vert the silver into a sulphate, as in the Ziervogel process. For the 
separation of silver from precipitated copper, J. A. Phillips, by a 
process patented in 1877, makes the material into a paste with water 
and a mixture of common salt and carbonate of soda; and, after 
calcination in a reverberatory, gets the silver in the form of chloride, 
which is dissolved out by brine from the oxidized copper. 

The solvent power of solutions of chloride of sodium for chloride 
of silver is diminished by dilution, and upon this fact, apparently, is 
based a process for the separation of silver, patented in 1877 in Great 
Britain by Jardine & Chadwick, which consists in diluting the strong 
lixivium from the salt-roasted ore to about specific gravity 1.10— 
1.12, and adding thereto, in the form of a dilute solution, about 
half a pound of acetate of lead to the ton of liquid. The precipi¬ 
tate, which after a time separates, consisting, in large part, of sulphate 
of lead, carries with it a portion of chloride of silver, and, it is said, 
a trace of gold. 

The most elegant method for the separation of silver from these 
mixed solutions is, however, that patented by Claudet, and exten- 

* 100 cc. of a solution holding 15.0 grams of chloride of sodium, dissolve at 
90° C., 10.0 grams ; at 40°, 6.0; and at 14°, 3.5 grams of cuprous chloride; while 
100 cc. of a solution holding 5.0 grams of chloride of sodium, dissolve at 90° 
2.6 grams, and at 40°, 1.4 grams of cuprous chloride. This substance, contrary to 
the received statements, is not quite insoluble in water When the cuprous 
chloride is boiled with distilled water an amount equal to about 1.35 grams to a 
liter passes into solution, and is in part thrown down on cooling in a white crys¬ 
talline form; the solution, at 14° C., still retaining about 0.90 grams to the 
liter. The above numbers are only approximations. 



THE HYDROMETALLURGY OF COPPER. 


5 


sively applied in Great Britain. It depends on the almost complete 
insolubility of iodide of silver in solutions of chloride of sodium, and 
consists in adding to the lixivium, in which the proportion of dis¬ 
solved silver has previously been determined, a dilute solution of a 
soluble iodide just sufficient in amount to convert the whole of the 
silver into iodide of silver. The precipitate which separates after 
forty-eight hours of repose, is washed with dilute chlorhydric acid 
to remove adherent copper-salts, and then consists chiefly of a 
mixture of sulphate of lead with iodide of silver, which is reduced 
by metallic zinc, the iodine being thus recovered for further use. 
Treated in this manner, the calcined Spanish ores yield to the ton 
20 grams of silver containing 1.3 per cent, of gold, amounting, ac¬ 
cording to Lunge, to about two-thirds the entire amount of precious 
metals contained in the ore. The presence of dichloride of copper 
in the solution interferes, by the production of a cuprous iodide, with 
the separation of the silver as iodide; and hence the calcination of 
the ores with salt must be so conducted as to give the copper in the 
condition of protochloride.* 

The extraction of copper from its ores by roasting with salt is 
limited to pyritous ores poor in copper, which yield, by their pre¬ 
vious calcination, a large proportion of peroxide of iron ; the pres¬ 
ence of this being necessary to the effectual chloridizing of the copper 
in the furnace. When applied to richer ores this method fails to 
render the whole of the copper soluble, for reasons which are made 
apparent bv the investigations of Mr. Thomas Macfarlane, described 
by him in 1865. He found that while copper-ores, such as chalco- 
pyrite and bornite, when calcined with salt, either alone or with an 
admixture of pulverized quartz, yield but a small portion of their 
copper in the form of soluble chloride, such a mixture of ore and 
salt, with twice its weight of peroxide of iron, and a little pyrites to 
furnish additional sulphur, if calcined at a low temperature, and 
without stirring, gave up nearly all its copper to water as a soluble 
chloride. It was made evident, from these and other experiments 
described by Macfarlane, that the mass of heated peroxide of iron, 
in the presence of air, favors the conversion of the sulphur into a 
sulphate, through which the decomposition of the common salt and 
the chloridizing of the copper is effected.f 

* For an excellent account of wet processes for the extraction of copper, see 
Friedr. Bode, in Dingler’s Polytechnisches Journal for January-March, 1877, 
vol. ccxxxi, pp. 254, 357, 428. 

f Canadian Naturalist, second series, vol. ii, pp. 219-231, and vol. iii, p. 457. 








6 


THE HYDROMETALLURGY OF COPPER. 


These conditions are most fully realized when a material like the 
calcined residue of Spanish pyrites, containing in 100 parts about 4 
parts each of copper and sulphur, and 80 parts or more of peroxide 
of iron, is calcined with a sufficient amount of common salt, in which 
case, as we have seen, the chloridizing of the copper is nearly com¬ 
plete. Ignorance of these conditions has more than once led to 
failure in attempts to apply this process of copper-extraction. 

Unlike the method of chloridizing by roasting with salt, that de¬ 
pending on the use of a solution of ferrous chloride with salt is a 
general one, applicable to all naturally or artificially oxidized copper- 
ores, which may be readily and cheaply chloridized by its aid.* 
When applied to copper ores containing silver, however, this shares 
with the salt-roasting process the disadvantage that the silver is at 
the same time chloridized, and if not present in too large an amount, 
is dissolved, while the dichloride of copper formed by the reaction 
between the oxide of copper and the ferrous chloride precludes the 
use of Claudet’s method of precipitating the dissolved silver by a 
soluble iodide. 

There is a large class of copper-bearing ores and furnace-products 
containing, besides silver, and in some cases gold, portions of anti¬ 
mony and arsenic, often accompanied by lead, the treatment of 
which, either by the wet or the dry way, offers many difficulties. A 
simple and economical general method, which will effect a complete 
separation of copper from silver and gold on the one hand, and from 
arsenic, antimony, and lead on the other, has hitherto been a desid¬ 
eratum in metallurgy. From the want of such a process considerable 
quantities of refined copper extracted from western ores and mattes, 
and carrying from 40 to 50 ounces of silver per ton, have of late years 
been sold in our markets. 

With my friend, Mr. James Douglas, Jr., of Phoenixville, Pa., I 
have devoted much time to the metallurgical problem thus pre¬ 
sented, and as the result of our joint labors have now to bring before 
the Institute of Mining Engineers a novel wet process for the ex¬ 
traction of copper from its ores, which will, I think, be found to 
meet the required conditions. The new method is based upon the 
reaction described by Wohler between sulphurous acid and a solu¬ 
tion of protochloride of copper, which gives rise to insoluble dichlo¬ 
ride with the elimination of one-half the chlorine in the form of 

* A process of copper-extraction, based on this principle, is the subject of 
United States letters-patent, granted to T. Sterry Hunt and James Douglas, Jr., 
February 9th, 1869. 






THE HYDROMETALLURGY OF COPPER. 


7 


chlorhydric acid, and the simultaneous formation of sulphuric acid, 
as simply expressed in the old notation by the formula 

2CuCl + S0 2 + HO = Cu 2 Cl + II Cl + S0 3 . 

The resulting acid solution, when brought in contact with cupric 
oxide, will take up as much copper as it originally held, which may, 
in its turn, be thrown down by sulphurous acid. In this way, the 
solution of copper from an oxidized ore, and its precipitation as 
dichloride, may be repeated indefinitely, provided chlorine be sup¬ 
plied each time by the addition of a sufficient amount of some solu¬ 
ble chloride. 

The reaction between sulphurous acid and a solution of proto¬ 
chloride of copper goes on slowly at ordinary temperatures, but is 
very rapid between 80° and 90° C. Solutions of sulphate of copper 
mixed with an equivalent of chloride of sodium, and holding 8.0 per 
cent, of copper, after being treated at 90° G. with an excess of sul¬ 
phurous acid gas, retain less 1.0 per cent, of dissolved copper; while 
in the presence of an excess of sulphate of copper and sulphurous 
acid the precipitation of the chlorine from chloride of sodium is 
nearly complete; sulphate of soda and sulphuric acid remaining in 
solution in accordance with the equation 

2Cu 0,S0 3 + NaCl + S0 2 = Cu 2 Cl + Na,S0 3 + 2SO s 

The sulphurous-acid gas for effecting this reaction on a large scale 
is readily got in sufficient purity from the burning of iron-pyrites in 
the ordinary kilns used by the makers of sulphuric acid, care being 
that an excess of air be avoided. A Knowles pump, constructed for 
the purpose, has proved an efficient means of injecting the heated 
gas into the liquid. By this reaction we have found it easy, in 
repeated trials with a small experimental plant, to throw down in 
three hours’ time 125 pounds of copper from an eight per cent, solu¬ 
tion, the liquid still retaining about one per cent, of copper dissolved. 

The acid liquors, when the reaction with sulphurous acid is com¬ 
plete, have exchanged their bright blue color for a pale green, and 
now contain in solution an excess of sulphurous acid, which must 
be got rid of before using them to dissolve a fresh portion of copper. 
This may be effected by keeping back a small portion of the chlori- 
dized copper-solution, and after the action of the gas is complete, as 
may be known by the changed color and the sulphurous odor of the 
liquid, adding the reserved portion thereto, by which means the 
excess of sulphurous acid will be oxidized. The chief part of the 
dichloride of copper separates during the passage of the gas, but n 
further portion is deposited on the cooling of the solution. 


8 


THE HYDROMETALLURGY OF COPPER. 


The excess of sulphurous acid may also be got rid of by blowing 
a current of hot air through the liquid after it has been withdrawn 
from the precipitated dichloride, and best while the process of sat¬ 
urating it with oxide of copper is going on. This, when got by the 
calcination of sulphuretted ores, contains more or less suboxide of 
copper,* which, with chlorhydric acid forms a portion of cuprous 
chloride, and the separation of this, under these circumstances, may 
be prevented by the action of atmospheric oxygen. 

Cuprous chloride is quickly transformed into cupric oxychloride by 
atmospheric oxygen, and when dissolved or suspended in an acid 
liquid is by this means readily converted into a cupric salt, which 
may be again reduced to cuprous chloride by the action of sulphurous 
acid. In this way, like the nitric oxide in the leaden chamber, the 
cuprous chloride acts as a medium through which sulphurous acid 
and oxygen are made to combine and to form sulphuric apid. The 
two reactions of oxidation and reduction just described may go on 
alternately or simultaneously in the liquid, and thus it happens that 
when an excess of air enters the pyrites-kiln, so that considerable 
free oxygen passes with the sulphurous acid into the copper-solution, 
the dichloride is either separated slowly or not at all, while at the 
same time much sulphuric acid is formed. By taking advantage of 
these reactions between oxygen, sulphurous acid, and chloride of cop¬ 
per, we may at will increase the solvent power of our acid bath. 

In applying this new process of copper-extraction to a roasted 
sulphuretted ore or matte, which we will suppose to contain a por¬ 
tion of silver, we begin by dissolving therefrom by water the sul¬ 
phate, which, with proper care in roasting, should contain not less 
than one-third of the copper of the ore; taking care to add to the 
water enough of some soluble chloride to chloridize and render 
insoluble any sulphate of silver which may be present. From the 
clear lixivium thus obtained, after adding the requisite amount of 
chloride of sodium, the copper is precipitated, as already described, 
by the action of sulphurous-acid gas. The resulting acid liquid, 
freed from the excess of sulphurous acid by the addition of a re¬ 
served portion of the original solution containing copper-chloride, 
and still retaining more or less copper, is now used to dissolve the 
oxide of copper from a portion of the lixiviated ore; the process being 

* I have found calcined sulphuretted copper-ores to contain, in addition to 
soluble cupric sulphate, and insoluble oxides of copper, a small portion of a 
cuprous compound, which, though insoluble in water, is dissolved by a hot and 
strong solution of common salt, and is probably a cuprous sulphate or sulphite. 




f 


THE HYDROMErALLURGY OF COPPER. 


9 


aided by heat, and, if the formation of dichloride of copper is to be 
feared, by the injection of a current of air, which may be made the 
means of heating and agitating the mixture. If the ore contains 
silver, either in the form of metal or unoxidized sulphide, we have in 
the chloride of copper which is formed the best agent for bringing it 
to the condition of chloride of silver. This will be found in the resi¬ 
due after the extraction of the copper, together with any gold which 
may be present, lead as sulphate, oxides of antimony and iron, and 
earthy matters. Cobalt, nickel, and zinc, if present will, however, 
be dissolved, and not being precipitated by sulphurous acid, will, by 
successive operations, accumulate in the solution, and may afterwards 
be extracted.* From the residues thus deprived of copper we have 
found the silver to be readily dissolved by brine,j* after which, if gold 
be present, it may be removed by chlorination, or the two precious 
metals may be extracted together from the residues by amalgamation. 
When, as in the case of certain mattes from Utah, for example, the 
residues contain a large amount of lead as sulphate, this may be 
recovered by smelting, and a base bullion got containing the precious 
metals. The same result may also be attained by smelting the res- 
sidues with an admixture of a lead-ore. 

Chloride of silver is soluble to some extent in solution of cupric 
chloride, and is then in part carried down with the cuprous chloride 
in the precipitation of the latter. The formation of cupric chloride 
may be avoided by adding to the solution of sulphate of copper 
little more than the amount of chloride of sodium necessary for the 
conversion of the copper into dichloride. In this case, as we have 
seen, the acid liquid after precipitation by sulphurous acid will con¬ 
tain chiefly sulphuric acid, though still holding sufficient cupric 
chloride to effect the chlordizing of any silver which may be present 
in the ore. 

The dichloride of copper, as obtained by precipitation, is a white 
coarsely crystalline powder, having a specific gravity of 3.376 (Play¬ 
fair and Joule), and, as we have seen (note on page 4), is nearly 
insoluble in cold water. After being washed from the acid liquid, it 
may be readily reduced by placing metallic iron in the moist di¬ 
chloride, which should be covered with water to exclude the air. 
The action spreads rapidly through the precipitate, so that a single 


* For observations on the association of nickel and cobalt with certain copper- 
ores, see Appendix I. 

f For notes on the solubility of chloride of silver in solutions of common salt 
and other chlorides, see Appendix II. 


9 







10 


THE HYDROMETALLURGY OF COPPER. 


mass of iron will, in a few hours, change a considerable volume of 
dichloride around it into pure spongy metallic copper. The reduc¬ 
tion of copper from solutions obtained in those wet processes where 
the copper exists as protochloride, often accompanied by salts of iron, 
entails a considerable loss of metallic iron, and gives a copper which 
is impure from the presence of basic iron-salts. The reduction of 
the solid dichloride, however, presents none of these disadvantages. 
Forty-five parts of iron suffice to reduce one hundred parts of cop¬ 
per; the precise ratio being as 28.0:63.4. The ferrous chloride 
which remains in solution may with advantage be used instead of 
chloride of sodium for chloridizing subsequent solutions of sulphate 
of copper, ferrous sulphate being formed which, as it accumulates, 
may be separated by crystallization from the acid liquid. The 
ferrous dichloride required to chloridize twenty parts of copper 
would equal about sixty-one parts of hydrated ferrous sulphate. 

Another mode of treating the dichloride, which may in some cases 
be resorted to, consists in decomposing it, best at a boiling heat, with 
a slight excess of milk of lime. The dichloride is by this means 
converted into a dense orange-red suboxide of copper which, after 
being washed from chloride of calcium, in a filter-press or otherwise, 
and dried, may be readily reduced to metallic copper in a reverbera¬ 
tory furnace. For this reaction, 28.0 parts of pure quicklime are 
required for 63.4 parts of copper, and the resulting chloride of cal¬ 
cium may be used instead of chloride of sodium or chloride of iron 
for chloridizing solutions of sulphate of copper. In this case, there 
will be formed an insoluble sulphate of lime or gypsum, while the 
free sulphuric acid of the solution is replaced by chlorhydric acid. 
The use of the chloride of calcium would, however, require an addi¬ 
tional operation, since, to avoid the presence of L the precipitated 
gypsum either with the dichloride or the undissolved residue of the 
copper-ore, it would be necessary to add the chloride of calcium to 
the clear copper-solution, and, after allowing time for the gypsum to 
subside, to transfer the liquid to the vats in which the copper is to 
be precipitated by sulphurous acid. There may, however, be local¬ 
ities in which the cost both of metallic iron and of common salt is 
such as to render advantageous the decomposition of the dichloride 
of copper by lime, provided there is no silver to be extracted. 

We have heretofore considered only the case in which the acid 
liquor got by precipitating the copper from neutral solutions in the 
form of dichloride is used to dissolve successive portions of oxide of 
copper alone. This can be done in the case of pure ores free from 


THE HYDROMETALLURGY OF COPPER. 


11 


other strongly basic oxides, if without loss, yet without any gain of 
acid save what comes incidentally from the portion of sulphuric 
anhydride which is given off in the calcination of pyrites, or from 
the reaction between sulphurous acid and oxygen in the presence of 
chloride of copper, as already explained. If, however, as is more 
often the case, we are treating artificially oxidized sulphuretted ores 
or mattes, which yield by roasting a mixture of oxide and sulphate 
of copper, it will be apparent that by the repeated use of the present 
process there must result a constantly augmenting proportion of free 
acid in the liquid. 

This may be made clearer by examples. Let us suppose a solution 
holding in a cubic foot (equal 1000 ounces of water) 63.4 ounces or 
two equivalents of copper in the form of sulphate. To convert this 
into protochloride would require two equivalents or 117 ounces of 
chloride of sodium, but for the production of the dichloride, as we 
have seen, one equivalent, or a little more, will suffice, or, in place 
thereof, a corresponding amount of ferrous or calcic chloride. When, 
by the action of sulphurous acid, the whole of the copper is reduced 
to the cuprous condition, and in a great part thrown down as di¬ 
chloride, the previously neutral solution will contain two equivalents 
or 98 ounces of sulphuric acid* (oil of vitriol), which, if a larger 
amount of chloride had been added, would be in part replaced by 
chlorhydric acid. These two equivalents of acid are capable of 
taking up two equivalents, or 79.4 ounces of oxide of copper, after 
which the solution will contain, as at first, 63.4 ounces of copper. 
If, however, we add to this acid solution, instead of simple oxide of 
copper, a calcined ore or matte in which one-third of the copper is 
present as soluble sulphate, and two-thirds as oxide, it is clear that 
when the acid is saturated we shall have in the liquid, besides the 
63.4 ounces of copper from the oxide one-half as much more, or 
31.7 ounces of copper which were already present as sulphate in 
the roasted ore; making in all three equivalents or 95.1 ounces of 
dissolved copper, which are, in their turn, to be converted into 
dichloride. Now, as the amount of acid set free in this reaction is 
equal to that originally combined with the copper, it follows that 
the liquid after the precipitation of the dichloride will contain three 

* While we recognize the dyad nature of copper, oxygen and sulphur, and 
the bibasicity of sulphuric acid, it is simpler and more convenient for the cal¬ 
culations of the manufacturing chemist and the metallurgist to use, as we have 
done in the present paper, the older notation, and to speak of 31.7 parts of cop¬ 
per, 8 parts of oxygen, 40 parts of sulphuric oxide, 49 parts of oil of vitriol, 36.5 
parts of chlorhydric acid, and 58.5 parts of chloride of sodium as equivalents. 





12 


THE HYDROMETALLURGY OF COPPER. 


equivalents of acid, instead of two as before. If to this we add, a 
second time, enough of the mixture of two-thirds oxide and one-third 
sulphate of copper to neutralize these three equivalents, we shall 
have four and a half equivalents of dissolved copper, from which, by 
a third repetition of the process of precipitation by sulphurous acid, 
four and a half equivalents of sulphuric acid would be set free; so 
that in place of 98 ounces we should have 220J ounces in the solu¬ 
tion ;—an amount which a fourth repetition of the process of satura¬ 
tion and precipitation would raise to six and three-quarter equiva¬ 
lents or 330 ounces of oil of vitrol. 

If, instead of a mixture containing one-third of its copperas sulphate, 
we have one in which only one-fourth is sulphate and three-fourths 
are oxide, we should get by saturating with this a solution con¬ 
taining two equivalents of acid, and subsequent precipitation with 
sulphurous acid, a liquid holding 2.66 equivalents of free acid, 
which by a third repetition of the process would yield 3.55, and by 
a fourth 4.73 equivalents of free acid, in place of the 2.00 equivalents 
which were present after the first precipitation. 

The above calculations are founded on the supposition that the 
roasted ore or matte contains, besides the oxide of copper, no base 
that would be attacked by dilute acids. In fact, however, oxides of 
lead, zinc and, more rarely, nickel and cobalt, may accompany the 
copper-oxide, and give rise, the first to an insoluble and the others 
to soluble sulphates, consuming more or less acid. Ores containing 
more or less carbonate of lime (often with carbonate of magnesia) 
are also of frequent occurrence, and here is seen a great advantage 
which this mode of copper-extraction possesses over all the other 
wet processes; for since lime and magnesia, and their carbonates, not 
only neutralize free acids, but throw down copper from its solutions, 
the treatment, by these processes, of ores containing any considerable 
proportion of calcareous matter is impracticable. With the process 
here proposed, which generates an abundance of free acid, the ex¬ 
traction of copper from ores which do not contain an excessive 
amount of calcareous matter presents no difficulty except such as 
arises from the mechanical obstacle created by the formation of 
gypsum in the solutions. The accumulation of acid in the bath is 
indeed so rapid in many cases that it will become unnecessarily 
strong, and may be diluted with water; while that portion not needed, 
after being deprived of the last portions of copper by the action of 
metallic iron, may be rejected unless it retains in solution other 
metals of value. 


THE HYDROMETALLURGY OF COPPER. 


13 


It will be seen from the foregoing description that the new process 
here described resembles those which, at the beginning of this paper, 
we have placed in Class II, inasmuch as the oxidized copper is sepa¬ 
rated from foreign metals by dissolving it in sulphuric and chlorhv- 
dric acids; with the difference, however, that the acids for this pur¬ 
pose are generated in the process itself, by the action of sulphurous 
acid, while the copper is separated from its solutions in the form of 
dichloride; the reduction of which to pure copper is readily effected 
by the consumption of a minimum amount of metallic iron. At the 
same time, any silver or gold which may be present in the ore is left 
undissolved, and in the best condition for subsequent extraction by 
well known methods, while the saving of cobalt and nickel, of lead, 
or of antimony, should these be present in quantities of economic im¬ 
portance, may be subsequently effected by very simple processes. 

The apparatus for this new general method of copper-extraction is 
simple and inexpensive. The chlorine required in the precipitation 
of the copper being recovered for further use, the only reagent con¬ 
sumed, except the sulphurous acid—which is a waste product 
from the roasting of sulphurous ores—is an amount of iron which is 
equal to less than one-half the weight of the copper, and may be 
recovered in the form of sulphate of iron,—or, instead thereof, the 
same quantity of caustic lime.* 

Appendix I. 

The presence of small portions of cobalt and nickel in cupri¬ 
ferous pyrites is not uncommon, and mixed earthy oxides of copper, 
nickel, and cobalt have been found in considerable quantities in Mis¬ 
souri. A greenish, translucent, amorphous mineral, with black 
stains, resembling chrysocolla in appearance, from some place in 
western Nevada, where it was said to be abundant, and to have 
been mined for the manufacture of sulphate of copper, was brought 
to me in 1876, and found to contain considerable quantities of both 
cobalt and nickel. One of two closely agreeing analyses by my 
former pupil, Mr. Hardman, made at the Mass. Institute of Tech¬ 
nology, in 1877, gave for this mineral as follows: Oxide of copper, 
9.63 ; oxide of nickel, 3.23 ; oxide of cobalt, 3.88 ; peroxide of iron, 
3.08; peroxide of manganese, 2.40; lime, 1.04; magnesia, 0.10; 


* United States letters-patent, No. 227,902, for this method of copper-extrac¬ 
tion were granted to Thomas Sterry Hunt and James Douglas, Jr., May 25th 
1880 . 





14 


THE HYDROMETALLURGY OF COPPER. 


alumina, 13.01 ; silica, 42.97; water, 18.38 — 97.72. The cobalt 
and nickel were separated by Rose’s method. Another analysis, in 
which these metals were separated by the method of Fischer, with 
nitrite of potassium, gave of oxide of cobalt, 4.11. Such an ore, if 
abundant, would be a valuable source of both nickel and cobalt. 

This aluminous mineral, like chrysocolla {ante, page 2) is attacked 
by a solution of ferrous chloride and common salt, by which the oxides 
of cobalt and nickel are indirectly dissolved ; since, although they 
have not the power of decomposing ferrous chloride, they decom¬ 
pose the cupric chloride which is formed by its reaction with cupric 
oxide. 


Appendix II. 

As regards the solubility of chloride of silver in solutions of chlo¬ 
ride of sodium, Vogel found that one liter of a saturated solution, at 
ordinary temperatures, held dissolved 0.950 grams of chloride of 
silver, while according to Hahn, a liter at 19.6° C. holds 1.269 
grams. Becquerel found at ordinary temperatures for a similar 
solution 0.800 grams to the liter. 100 parts of water, saturated at 
100° C., hold 26.61 parts, and at 15.6° C., 26.34 parts of chloride 
of sodium, the densities of the solutions being respectively 1206.93 
and 1204.03. Hence, one liter of a saturated solution at 15.6° 
holds 316 grams of common salt, 1000 parts of which solution 
under these conditions dissolve, according to Hahn, at 15.6° C., 3.0 
parts of chloride of silver; while, according to the observations of 
Vogel and of Becquerel, at “ ordinary temperatures,” not defined, 
1000 parts, in saturated solution, dissolve respectively 4.0 parts and 
2.53 parts of chloride of silver. The latter figure approximates to 
that given by Pelouze and Fremy, according to whom 1000 parts of 
salt at 18° C. hold dissolved 2.40 parts of chloride of silver. The 
solvent power, according to these chemists, varies greatly with the 
temperature, the amount dissolved being equal to 1.70 parts at 10° 
C., and not less than 6.80 at 100° C., while at 0° C. but traces of 
chloride of silver are dissolved. Differences of temperature may 
suffice to explain the discrepancies between the results of Vogel, 
Hahn, and Becquerel, but not those of Pelouze and Fremy at 18.0° 
C., a temperature above that mentioned by Hahn. It is possible 
that these chemists may not have employed solutions saturated with 
chloride of sodium, to which the observations of the others refer. 
Fresenius, speaking of the solubility of chloride of silver in hot 
concentrated solutions of the chlorides of sodium, potassium, am- 


THE HYDROMETALLURGY OF COPPER. 


15 


monium, calcium, zinc, etc., says “ On sufficient dilution with cold 
water the dissolved portion separates so completely that the filtrate 
is not colored by sulphuretted hydrogen.”* 

As to the solubility of chloride of silver in some other chlorides, 
Hahn found that a liter holding 30.70 per cent, of ferrous chloride, 
and having a specific gravity of 1.419, dissolves, at 20° C., 2.385 
grams of chloride of silver; while a solution, holding 44.48 per cent, of 
cupric chloride, and having a specific gravity of 1.5726, dissolves at 
30° C., for 1 liter, 0.836 grams of chloride of silver. For farther 
observations on the solubility of chloride of silver in other chlorides, 
see Percy, Metallurgy of Silver and Gold, Part I, p. 58, and also 
Plahn, Transactions American Institute Mining Engineers , Vol. II, 
p. 99. 


* Fresenius, Quantitative Analysis, Amer. Ed., 1879, p. 124. 













































